Soils vary in the stability of their structural porosity in wet conditions. As wet soil disaggregates the soil consolidates to a denser and smaller volume.
At depth, soils are subject to increasing overburden pressure from above due to the weight of soil and the associated water content. Furthermore, the internal friction between soil particles decreases with wetness, reducing resistance to consolidation.
Such processes cause cultivated soils to consolidate over time into denser and less permeable states in which water and air movement and root growth are restricted, reducing shoot growth and grain or forage production. Settled disaggregated soil is ill suited for the germination and growth of plants.
Loose soil, on the other hand, facilitates the growth and proliferation of roots, which increases soil organism populations, both of which stabilise the soil structure and increase crop production. In addition, roots act as reinforcing rods, binding and separating soil particles to withstand the processes of disaggregation and consolidation.
Conventional cultivation inverts soil and tears out root systems exposing the roots, organic matter and soil organisms to desiccation and irradiation by ultra-violet light. As a consequence, cultivated soils have less roots, organic matter and soil organisms and are therefore less stable when wet. As such, conventionally cultivated soils rapidly disaggregate and consolidate to densities that constrain plant growth and development.
The advent of selective herbicides and no-tillage seeders has allowed for the reduced cultivation of narrow strips of soil in which seed and fertiliser are placed. Whilst this practice has generally led to increases in organic matter and stability of the surface layer of soil, it does not prevent settlement and consolidation of soil between the sown rows and below about 80 mm depth.
Furthermore, irrigated soils are particularly prone to consolidation due to frequent applications of large amounts of water that wet the soil close to saturation point. Similarly, soils in high rainfall areas may experience saturation and consolidation when rainfall exceeds evaporation and drainage.
Current best practice and technology recommends farmers irrigating or growing rainfed no-tillage crops on soils with consolidated root zones below about 80 mm. These consolidated root zones limit drainage of waterlogged soils and slow infiltration on irrigated soils. In consequence, irrigation applications need to be prolonged to allow time for water to penetrate to the centre of beds to ensure even crop growth.
In both sets of circumstance however, production is constrained, and in the case of irrigation, prolonged application times lead to considerable amounts of water being ‘lost’ to drainage beyond the root zone of crops. In fact, industry standards for irrigation accept that 150 mm of water is lost to deep drainage each cropping season, being equivalent to 1.5 mega litres/hectare. With water in the Murray-Darling Basin of Australia costing $1,500 to $3,000 a megalitre of water, this equates to a cost of $2,250 to $4,500 per hectare per crop.